Suppose you know the area of the yard 7,500 square feet how can you find the value of a

Unfortunately, you have not defined "a."  Is "a" the length or the width of the 7500-sq-ft yard?  If "a" represents the length, then the width would be 

7500 sq ft
= width
       a

Please ensure that you have shared ALL of the given problem.

Ithink it would be 160 people